1. Field of the Invention
This invention relates generally to bearing tolerance rings and, more specifically, pertains to tolerance rings used in cartridge bearings for actuator arms in information storage devices.
2. Description of Prior Art
A key component of any computer system is a device to store data. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations on the disc, and electrical circuitry that is used to write and read data on and from the disc. There are a variety of disc drives in use such as hard disc drives, zip drives, floppy disc drives which all utilize either rotary or linear actuators.
In disc drive systems, magnetic heads read and write data on the surface of co-rotating discs that are coaxially mounted on the spindle motor. The bits of information written on a disc are laid out in concentric circular tracks on the surface of the discs. The discs must rotate quickly so that the computer user does not have to wait long for a desired bit of information on the disc surface to translate to a position under the head. In modern disc drives, especially in hard disk drives, data bits and tracks must be extremely narrow and closely spaced to achieve a high information density per unit area of the disc surface.
The required small size and close spacing of information bits on the disc surface have the consequences on the design of the disc drive device and its mechanical components. The most important consequence is the magnetic transducer on the head must operate in extremely close proximity to the magnetic surface of the disc. Because there is relative motion between the disc surface and the head due to the disc rotation and head actuation, continuous contact between the head and disc can lead to failure of the interface. Such failure can damage the disc and head and usually causes data loss. To avoid this problem, a magnetic head is typically designed to be hydrodynamically supported by an extremely thin air bearing surface (“ABS”). When a disc rotates, air is dragged between the head and the disc surface, causing pressure, which forces the head away from the disc. At the same time, the air rushing past the head and disc produces a negative pressure area. These forces are designed to balance so that the magnetic head flies over the surface of the disc at a particularly desired fly height in very close proximity to the disc while avoiding physical contact between the head and disc. In typical systems, the spacing between the head and disc during operation is extremely small, measuring in the tens of nanometers.
Another consequence of the close spacing required between the bits and tracks written on the disc surface is that the spindle rotation and head actuator motion must be operated with extremely high precision. The head actuator must have structural characteristics that allow it to be actively controlled to quickly seek different tracks of information and then precisely follow small disturbances in the rotational motion of the disc while following the tracks. The characteristics of the actuator structure that are important to this end, include stiffness, mass, geometry, and boundary conditions. For example, one important boundary condition is the rigidity of the interface between the actuator arm and the actuator pivot bearing.
All structural characteristics of the actuator must be considered by the designer to minimize vibration in response to rapid angular motions and other excitations. For example, the actuator arm cannot be designed to be too massive because it must accelerate very quickly to reach information tracks containing desired information. Otherwise, the time to access desired information may be unacceptable to the user. On the other hand, the actuator arm must stiff enough and the actuator pivot bearing must be of high enough quality so that the position of the head can be precisely controlled during operation. The interface between the actuator arm and the pivot bearing must be of sufficient rigidity and strength to enable precise control of a head position during operation and to provide the boundary conditions necessary to facilitate higher natural resonant frequencies of operation of the actuator arm structure. Actuator arm stiffness must also be sufficient to limit deflection that might cause contact with the disc during mechanical shock events that may occur during operation or non-operation. The interface between the actuator arm and the pivot bearing must be of sufficient strength to prevent catastrophic structural failure such as actual slippage between the actuator arm and the actuator pivot-bearing sleeve, during large mechanical shock events.
In many disc drives, the actuator arm or arms are fixed to the actuator pivot bearing by a tolerance ring. Typically, tolerance rings include a cylindrical base portion and a plurality of contacting portions that are raised or recessed from the cylindrical base portion. The contacting portions are typically partially compressed during installation to create a radial preload between the mating cylindrical features of the parts joined by the tolerance ring. The radial preload compression provides frictional engagement that prevents actual slippage of the mating parts. For example, in disc drive applications, the radial compressive preload of the tolerance ring prevents separation and slippage at the interface between the actuator arm and the pivot bearing during operation and during mechanical shock events. The tolerance ring also acts as a radial spring. In this way, the tolerance ring positions the interior cylindrical part relative to the exterior cylindrical part while making up for radii clearance and manufacturing variations in the radius of the parts.
Additional features have been added to tolerance rings to obtain the specific advantages. For example, in U.S. Pat. No. 6,288,878 to Misso et al., circumferential brace portions have been added to the tolerance ring to increase hoop strength. U.S. Pat. No. 6,338,839 to Misso et al. discloses a tolerance ring which provides a low consistent installation force profile.
U.S. Pat. No. 4,790,683 to Cramer, Jr. et al. discloses the use of a conventional tolerance ring in conjunction with a cylindrical shim in applications characterized by structurally significant radial vibration or loading. The shim prevents deformation of the soft underlying material and thereby prevents undesirable partial relief of the radial compression that maintains frictional engagement of the tolerance ring.
State of the art tolerance rings are typically manufactured from a flat metal sheet with stamping, forming, rolling, and other steps to provide ways to recess contacting portions and a final generally cylindrical shape. The tolerance ring can be installed first into a generally cylindrical hole in an exterior part, such as an actuator arm, so that later a generally cylindrical inner part, such as an actuator pivot bearing, can be forcibly pushed into the interior of the tolerance ring to create a radial compressive preload that retains the parts by frictional engagement. In this case, the contacting portions may be recessed to a lesser radius than the base portion as well as raised to a greater radius than the base portion. Alternatively, a tolerance ring can be installed first around a generally cylindrical inner part, such an actuator pivot bearing. The inner part, together with the tolerance ring, is then forcibly pushed into the interior of the generally cylindrical hole in an exterior part, such as an actuator arm, to create a radial compressive preload that retains the parts by frictional engagement. In this case, the contacting portions of the tolerance ring are typically raised to a greater radius than the base portion.
There is a need in the art for a tolerance ring that can accommodate thermal mismatches that might occur between the bearing cartridge and the actuator arm as the disc drive heats from a starting temperature to an operating temperature. Moreover, there is a need for a tolerance ring that provides an increased internal diameter static friction that not only provides better performance, but prevents the tolerance ring from slipping during operation as a result of a shock event.